Quando lo yogurt è troppo liquido, aggiungo più cereali e meno miele.

Yogurt is usually treated as a masculine singular noun in Italian. In standard usage, many foreign words beginning with y take lo, so lo yogurt is very common.

You may also hear il yogurt in everyday speech, but lo yogurt is a very standard choice for learners.

The accent marks it as the verb è = is.

This helps distinguish it from e = and.
So:

• è = is
• e = and

Italian usually drops subject pronouns when they are not needed. The verb ending already tells you who is doing the action.

So aggiungo already means I add.
You would use io aggiungo only if you want emphasis, contrast, or extra clarity.

This sentence describes a habitual or general action: whenever this situation happens, this is what I do.

Italian normally uses the present indicative for that:

• Quando lo yogurt è troppo liquido...
• ...aggiungo più cereali e meno miele.

So the present tense here means something like Whenever the yogurt is too runny, I add...

Here troppo is an adverb meaning too, and in Italian it normally goes before the adjective it modifies.

So:

• troppo liquido = too liquid / too runny

Also, because troppo is acting as an adverb here, it does not change form. It stays troppo, not troppa or troppi.

After forms of essere, Italian uses an adjective to describe the subject. This is very normal:

• Lo yogurt è liquido
• Il caffè è caldo
• La minestra è fredda

So liquido is simply the adjective describing yogurt.

Because the sentence is talking about an indefinite amount, not specific cereal or specific honey.

After più and meno, Italian often uses the noun without an article:

• più cereali
• meno miele
• più acqua
• meno zucchero

If you added an article, it would usually sound more specific or refer to a particular known amount or item.

When più or meno is followed directly by a noun, you usually do not use di:

• più cereali
• meno miele

You use di when comparing with something else:

• più di prima = more than before
• meno di ieri = less than yesterday

So in this sentence, più and meno are directly modifying the nouns.

In Italian, cereali is commonly used in the plural when talking about breakfast cereal or cereals in general.

Miele, on the other hand, is a mass noun, so it is normally singular, like honey in English.

So this pattern is very natural:

• più cereali
• meno miele

It is very normal and usually recommended here because the sentence begins with a subordinate clause:

• Quando lo yogurt è troppo liquido, aggiungo...

The comma helps separate the when-clause from the main clause.
If you reversed the order, the comma would often be omitted:

• Aggiungo più cereali e meno miele quando lo yogurt è troppo liquido.

Yes, if the context already makes it clear that you are talking about the yogurt.

For example:

• Quando è troppo liquido, aggiungo più cereali e meno miele.

That sounds natural if everyone already knows what it is.
But Quando lo yogurt è troppo liquido... is clearer on its own, especially as a standalone sentence.

Liquido is correct and easy to understand. In everyday speech, some people might also say troppo fluido or describe yogurt as troppo poco denso, depending on the nuance.

But troppo liquido is perfectly natural and clear for learners.